X-Ray
Point Source Excess of MACS Clusters

Joshua Ruderman
Mentor: Harald
Ebeling

Researchers have published several papers over the past
several years
discussing the apparent excess of X-ray point sources,
beyond the background
rate, in Chandra images of clusters of galaxies. These
X-ray point sources are
believed to be Active Galactic Nuclei (AGN) members of
the cluster which emit
X-rays due to Compton scattering in the accretion disk.
Another possibility is
that X-ray point sources are due to gravitational lensing
of background
galaxies. All previous studies have looked at only one
to two clusters and
therefore have poor statistics for characterizing the distribution
of this
point source excess. This project is a study of the X-ray
point sources of 40
clusters, which range in redshift from z = 0.3
to 0.6, improving on past statistics
by an order of magnitude.

After reprocessing the raw data to remove instrumental
artifacts and background
flares, the Celldetect algorithm from the CIAO software
package is used to
detect point sources with a 3 σ signal-to-noise
ratio above the
background, resulting in 719 point source detections in
the 40 clusters.
Source count rates are converted to unabsorbed energy fluxes
(0.5–10 keV) using
X-Spec software and a scaled power law source spectral
model. Luminosities are
similarly computed in the 0.5–10 keV range, accounting
for K-correction. Based
on NLog-SLog graphs and source exposure times, a global
luminosity limit of
2.4 e43 erg/sec is established, leaving 235
X-ray point sources. Using the
assumption that the point sources are at the cluster redshift,
the sources are
binned into annuli of varying distances from the center
of the diffuse cluster
emission. The spatial detection density is determined to
have 5.7, 3.1, and
2.1 σ excesses out to 3.33 Mpc for the z =
0.5–0.6, 0.4–0.5, and 0.3–0.4 clusters,
grouped separately, indicating cluster evolution. The
first statistically
significant profile of the sources is shown to have a continuous
profile peaked
at the diffuse emission center and reaching the background
level at a distance
of 5 Mpc from the cluster center.